The present invention relates to an electro-hydraulic control for varying the control pressure to a standard controller used to port fluid to the servos of a variable displacement pump or motor.
It is common in designing controls for hydrostatic transmissions to design a distinct control for each desired function. For example, a pressure override (P.O.R.) control is designed to monitor the system pressure of a transmission to protect the transmission from extending excessive overloads. P.O.R. controls are well known in the art and will be disclosed, in part, in the present application. An anti-stall control is used to destroke the swashplate of a pump in response to loading of the prime mover of the pump. Anti-stall controls generally make use of governors (see U.S. Pat. Nos. 2,516,662 and 2,976,685) to directly control movement of the valve spool and thereby control fluid pressure. Another type of control is a phasing control which is used to first increase the displacement of the pump swashplate in a hydrostatic transmission to its maximum and then decrease the displacement of the motor swashplate to a minimum to achieve an increase in speed of the transmission, and to reverse the process for a decrease in speed. Phasing controls generally make use of cams (see U.S. Pat. No. 2,516,662).
Yet a further control is an input torque limiter (I.T.L.) control which matches the torque of a hydrostatic transmission to that of the prime mover. I.T.L. controls generally make use of cams to reset the compensating override pressure for each swashplate position to maintain constant the product of system pressure and pump displacement. Other known I.T.L. controls are hydraulic wherein a pressure drop across a compensating or override spool is maintained proportional to the pump displacement. This is generally accomplished by a variable orifice. Further, other known I.T.L. controls are electrical. In the electrical I.T.L. controls the displacement of the pump and the system pressure are each measured and then multipled to produce a signal which is then used to control the displacement of the pump. All of the electrical I.T.L. controls of which applicant is aware make use of a pressure transducer.
Although each of the preceding referenced controls perform their respective functions generally satisfactorily, they are relatively cumbersome, complicated, difficult to adjust, and expensive. Further, a separate, distinct control exists for each of the functions. Generally, the parts of each of the controls cannot be interchanged with parts from another control.
Accordingly, it is an object of the present invention to provide a simple, inexpensive control which may be readily adapted to perform a number of functions in controlling the operation of a variable displacement pump, a variable displacement motor, or a hydrostatic transmission including a pump and motor combination.
It is a further object of the present invention to provide a basic component which may be simply and easily adapted to control any one of a number of functions of either a hydrostatic transmission or the pump or motor of the transmission.
In many of the electrical controls of the type discussed above, it is desirable to generate a command signal which is a quotient resulting from the division of one system characteristic by another, wherein either the dividend or the divisor may be a manually selected input, or may be a variable system characteristic. For example, in the electro-hydraulic I.T.L. control disclosed hereinafter, the system logic generates a variable pressure command signal corresponding to maximum system pressure for any given displacement. Mathematically, this pressure command signal is obtained by dividing the maximum input torque (a manual setting) by the pump displacement (a variable system characteristic).
Known electro-hydraulic controls have accomplished necessary division functions in generally one of two ways: either the quotient is approximated, using a linear approximation (or a series of linear approximations), or the quotient is actually calculated by means of an analog divider. The linear approximation method results in relatively poor performance, while the analog division method is excessively expensive for application in hydrostatic transmission controls.
Accordingly, it is another object of the present invention to provide an electro-hydraulic control including a simple, inexpensive method of calculating a "quotient" which is then used by the control as a command signal.